1. Field of the Invention
The present invention relates generally to superconducting devices and more specifically to superconducting devices modulated by a controllable weak link.
2. Description of the Background Art
Superconducting materials have found uses in devices such as amplifiers, logic devices, Josephson junctions and switches. For superconducting materials to be useful in these and other devices, it is desirable to control or otherwise modulate their superconducting properties. This control has been typically accomplished by forming a weak link in the superconducting material. At a weak link, the ability of the superconducting material to carry current is significantly reduced with respect to the remaining bulk of the superconductor.
Weak links can be fixed or variable. A weak link formed by reducing the width and/or thickness of a selected portion of a superconducting film is fixed. That is, the size of the weak link cannot be readily altered. Other fixed weak links include superconductor-normal (non-superconducting) metal-superconductor junctions and the superconducting point contact.
Fixed weak links have also been formed by overcoating a narrow strip of a superconductor with a non-superconducting metal. The non-superconducting metal weakens the superconductivity of the overlaying portion of the superconductor.
Variable weak links, in which the characteristics of the weak link can be readily adjusted, are desirable because they offer the ability to modulate the superconducting current across the link. Previously, variable weak links have been created by injecting current from a non-magnetic and non-superconducting metal into a small region along a superconducting wire, strip or film. These unpolarized carriers minimally disrupt the singlet ground state (paired spin up and spin down carriers) in the superconductor. Also, the injection of unpolarized carriers changes the relative population of spin-paired and unpaired carriers, thus reducing the critical current of the superconductor.
The reduction in superconducting current created by this conventional injection of non-spin polarized carriers, however is small and the gain is therefore low. A superconducting device in which the size and critical current density of the weak link can be better modulated and/or tuned and thus provide higher gain is desirable.
Others have shown that, in a bilayer consisting of a spin-polarized material and a conventional superconductor, the polarization of the spin-polarized carriers penetrates the interior of the superconductor. Previously, however, no one has suggested the injection of spin-polarized carriers, or the use of other spin-polarization phenomena, to suppress the order parameter, and thus control the conductivity, of a superconductor.